Interactions between spacecraft and a space plasma environment can cause large electrical potential differences to develop between spacecraft structures and the space plasma environment. If uncontrolled, the charging of the spacecraft can produce enhanced surface contamination, increased power leakage currents, and electrical arcs that cause system performance degradation or failure.
Currently available spacecraft charging detection and measurement instruments include floating potential probes, charged-particle energy spectrum analyzers, and electrostatic surface potential monitors. Active charge control techniques include thermionic electron emitters, neutral gas release systems, and plasma generators. A typical charge control device is generally referred to a plasma contactor. This device employs an electrically driven hollow cathode system to ionize and then release an expendable gas such as argon, xenon, or krypton from an onboard gas supply tank. The resulting plasma cloud, which is generated essentially at the spacecraft""s potential, allows charged particles of the appropriate polarity to flow to the ambient plasma environment, thus reducing the absolute spacecraft potential.
Disadvantages with this type of charge control device include the weight and complexities associated with the gas storage and mechanical gas flow control subsystem and the time limits placed on operating by the finite gas supply. Also, to maximize system effectiveness and prevent the unnecessary use of gas, a separate supplementary charging monitor system is required to determine when to turn the plasma contactor on and off. Thus, there remains a need for a system and method for controlling charge on a spacecraft without the disadvantages of the devices described above.
One objective of the invention is to provide a charge emitter that is suitable for use in space applications, i.e., a charge emitter that is lightweight, small in size, and reliable and robust in the space plasma environment. Another objective is to provide a charge emitter capable of emitting low levels of charge, positive or negative, for controlling the charging of a space object. Another objective is to provide a charge emitter capable of emitting large levels of charge, positive or negative, for use in adjusting the orbit of space objects. Yet another objective is that the charge emitter be capable of emitting charge without requiring high voltages. Still yet another objective is that the charge emitter be capable of emitting electrons without the use of expendable resources.
The invention features a controller for controlling an electrical potential of an object with respect to its environment. The controller comprises a charge-emitting device connected to an object situated in an environment. The charge-emitting device has a gate and an emitter and appropriate exposure to the environment of the object. A voltage source applies a voltage across the gate and the emitter of the charge-emitting device to induce the emitter to emit into the environment electrical charge that is drawn from the object. A control system, in communication with the voltage source, varies the voltage applied between the gate and the emitter of the charge-emitting device to adjust an amount of electrical charge emitted by the emitter so as to control the electrical potential of the object with respect to the environment.
In one embodiment, the gate is electrically connected to the object and power for controlling the electrical potential of the object with respect to the environment is delivered by the voltage source. In another embodiment, the emitter is electrically connected to the object, the object is being charged by a charge source, and power for controlling the electrical potential of the object is obtained from the charge source. A switch located between the charge-emitting device and the object can connect one of the gate and the emitter to the object.
The object can be connected to a source of charge. The charge source that charges the object can be a natural charging phenomenon that occurs when the object interacts with the environment. In one embodiment, the charge source is an electrodynamic conductive tether moving through a magnetic field. The electrical potential of the object with respect to the environment increases, decreases, or is substantially unchanged, depending upon whether the amount of emitted charge is greater than, less than, or approximately equal to the net amount of charge of the same polarity as the charge received and emitted by the object.
In another aspect, the invention features an apparatus for emitting charge from an object. The apparatus includes a charge-emitting device in electrical communication with the object to draw charge from the object. The charge-emitting device has at least two terminals. A first voltage source applies a voltage across the two terminals of the charge-emitting device to induce one of the terminals to emit charge drawn from the object. The magnitude of the voltage applied by the first voltage source can be less than 100 volts. A second voltage source applies a voltage between one of the two terminals and the object, to connect that one terminal to the object and to determine a path through the charge-emitting device taken by the charge that is drawn from the object and emitted by the emitting terminal. The voltage applied by the second voltage source can be positive, negative, or zero volts. Zero volts can be achieved by directly connecting the one terminal to the object such as by a wire. The emitted charge can be positive or negative in polarity.
The two terminals of the charge-emitting device are a gate and an emitter. In one embodiment, the gate is the one terminal connected to the object through the second voltage source. The path of the charge goes to the emitter through the first voltage source. In another embodiment, the emitter is the terminal connected to the object through the second voltage source. The path of the charge goes to the emitter without passing through the first voltage source.
In yet another aspect, the invention features a system for emitting charge from a space object. The system includes a micro-fabricated charge-emitting device connected to a space object situated in a space plasma environment. The charge-emitting device has a gate and an emitter in close proximity to the gate. A voltage source applies a voltage with a magnitude of less than 100 volts across the gate and the emitter of the micro-fabricated charge-emitting device to induce the emitter to emit electrical charge that is drawn from the space object into the space plasma environment. The electrical potential of the object with respect to the environment increases, decreases, or is substantially unchanged depending upon the net charge that is emitted into the space plasma environment.